MORPHOGENESIS OF FRESH-WATER PLANARIA
327
Furthermore, if the regeneration of fresh-water planarians is related
to the presence of healthy neoblasts, it is appropriate to inquire whether
these cells are present in planarians which are incapable of regeneration.
Stephan-Dubois (1961) noted that the density of neoblasts of the
Dendrocoelida, which have limited capacity for regeneration, is lower
than in Dugesia lugubris. In the marine polycladian, Leptoplana
tremellaris, which shows little capacity for regeneration, Lender (unpublished) found cells in the parenchyma whose cytoplasm was rich in
RNA and which stained with methyl green-pyronine.
Not all animals capable of regeneration possess omnipotent regeneration cells. But there is a pronounced analogy between the neoblasts of
the planarians and the interstitial cells of the coelenterates. The results
obtained by Tardent (1954) on Hydra and Tubularia confirm this point
of view.
A study of the factors of organ differentiation points to the fact that
here too our knowledge is very fragmentary. It is now known that
hormonal correlations exist in the Planaria.
The formation of the copulatory apparatus probably depends on the
activity of hormonal substances emitted by the genital glands, at least
in the experiments on sexualization of asexual planarians.
The analysis of organ differentiation in the Planaria also shows up
induction phenomena. The brain induces the regeneration of the eyes
by means of chemical substances which diffuse in the body of the animal.
The anterior region of the planarian induces a pharyngeal region which
then acquires the faculty of differentiating a pharynx. A double induction is necessary in this instance of regeneration.
These induction phenomena emphasize again the parallelism which
exists between the process of differentiation during regeneration and
embryonic development. In both instances, a privileged area emits
organizers capable of inducing the differentiation of an organ from
undifferentiated cells.
During regeneration of the Planaria, yet another type of correlation
can be demonstrated: inhibition. The cephalic region containing the
brain inhibits the regeneration of another brain. A regeneration
blastema is formed by accumulation of neoblasts it grows, but the
differentiation of nerve cells is inhibited. In this instance the inhibitory
activity of the brain is also exerted by means of chemical substances.
The study of inductive or inhibitory substances secreted by different
organs of the planarians is still in its early stages.
The substance of induction of sexual differentiation and of regeneration of the pharynx is still unknown. But it is recognized that the brain
secretes at least two types of substances, those which inhibit the
regeneration of the brain and the Organisines' which induce the
327
Furthermore, if the regeneration of fresh-water planarians is related
to the presence of healthy neoblasts, it is appropriate to inquire whether
these cells are present in planarians which are incapable of regeneration.
Stephan-Dubois (1961) noted that the density of neoblasts of the
Dendrocoelida, which have limited capacity for regeneration, is lower
than in Dugesia lugubris. In the marine polycladian, Leptoplana
tremellaris, which shows little capacity for regeneration, Lender (unpublished) found cells in the parenchyma whose cytoplasm was rich in
RNA and which stained with methyl green-pyronine.
Not all animals capable of regeneration possess omnipotent regeneration cells. But there is a pronounced analogy between the neoblasts of
the planarians and the interstitial cells of the coelenterates. The results
obtained by Tardent (1954) on Hydra and Tubularia confirm this point
of view.
A study of the factors of organ differentiation points to the fact that
here too our knowledge is very fragmentary. It is now known that
hormonal correlations exist in the Planaria.
The formation of the copulatory apparatus probably depends on the
activity of hormonal substances emitted by the genital glands, at least
in the experiments on sexualization of asexual planarians.
The analysis of organ differentiation in the Planaria also shows up
induction phenomena. The brain induces the regeneration of the eyes
by means of chemical substances which diffuse in the body of the animal.
The anterior region of the planarian induces a pharyngeal region which
then acquires the faculty of differentiating a pharynx. A double induction is necessary in this instance of regeneration.
These induction phenomena emphasize again the parallelism which
exists between the process of differentiation during regeneration and
embryonic development. In both instances, a privileged area emits
organizers capable of inducing the differentiation of an organ from
undifferentiated cells.
During regeneration of the Planaria, yet another type of correlation
can be demonstrated: inhibition. The cephalic region containing the
brain inhibits the regeneration of another brain. A regeneration
blastema is formed by accumulation of neoblasts it grows, but the
differentiation of nerve cells is inhibited. In this instance the inhibitory
activity of the brain is also exerted by means of chemical substances.
The study of inductive or inhibitory substances secreted by different
organs of the planarians is still in its early stages.
The substance of induction of sexual differentiation and of regeneration of the pharynx is still unknown. But it is recognized that the brain
secretes at least two types of substances, those which inhibit the
regeneration of the brain and the Organisines' which induce the